Theres been a few issues around Manifest#compile targeting assets using transformers. https://github.com/josh/sprockets-es6/issues/9 Sprockets 3.0.0.rc QA #10 (comment) The issue has to do with how 2.x and 3.x implement manifest.compile. To support arbitrary procs and fnmatch syntax, sprockets has to iterate over every darn file in the load path to check it against these filters. Its terribly slow